1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a multicast or broadcast method, and a device supporting the method in a wireless local access network (WLAN).
2. Discussion of the Related Art
Recently, diverse wireless communication technologies have come under development in line with the advancement of information communication technology. The wireless local access network (WLAN) technologies allow mobile terminals such as personal digital assistants (PDAs), laptop computers, portable multimedia players (PMPs), and the like, to wirelessly access the high speed Internet at homes or offices or in a particular service providing area based on a radio frequency technology.
In a WLAN, message or frame transmissions may be divided into a unicast transmission, a multicast transmission, and a broadcast transmission depending on the number of target devices or destination devices. Whereas the unicast transmission is aimed for only a single target device of a transmission frame, the multicast and broadcast transmissions are used when a plurality of terminals are target devices of a transmission message. In multicast, a target address or a destination address of a transmission frame is specified as a multicast group address, and broadcast is a special multicast in which the multicast group address specifies every terminal within a certain coverage. Thus, although ‘multicast’ is simply referred to hereinafter, it should be construed to include ‘broadcast’ except for a case not allowed in its properties.
Multicast simultaneously transmits the same frame to a plurality of destination terminals, having an advantage of reducing traffic and effectively using channels. With such advantages, multicast may be effectively used for various applications, for example, providing video conference, corporate communication, distance learning, software distribution, or various information such as stock quotes, news items, and the like. Further, multicast may be used for an application allowing a plurality of users to play games or share streaming data in wireless home network.
Multicast is based on a group of reception terminals interested in a particular data stream, namely, based on the concept of a multicast group. If terminals are interested to receive multicast data, the terminals should first join a corresponding multicast group to receive the data. The multicast group is specified by a multicast medium access control (MAC) address in a MAC layer. In general, an upper layer of the MAC layer is accountable for generating, joining, seceding, and changing of the multicast group.
Meanwhile, in the multicast service, it is difficult to confirm whether or not a terminal joined the corresponding multicast group, namely, a destination terminal, has successfully received every data provided from a source terminal. In particular, IEEE (Institute of Electrical and Electronic Engineers) 802.11 standard does not define an error recovery mechanism with respect to multicast traffic. Namely, the current IEEE 802.11 standards use an open-loop multicast transmission, without receiving any feedback information from the reception terminal regarding the multicast traffic. As a result, in the current WLAN, the reception terminal, namely, the target terminal, of the multicast traffic, it is not guaranteed to completely and reliably receive data transmitted in the multicast manner.
In addition, the current WLAN standard does not support retransmission, exponential backoff, and physical layer (PHY) rate adaptation with respect to a multicast frame. For example, in the case of the unicast transmission, whether or not a frame has been received is checked through an acknowledgement (ACK) frame. If no ACK frame is received, the source terminal transmits the same frame again. In this case, a contention window is exponentially increased to achieve fair throughput with other neighbor terminals, and the source terminal adjusts the physical layer rate based on a frame error rate. In contrast, however, the current multicast service does not support protocols comparable to those used in a unicast transmission, thus having a difficulty in providing quality of service (QoS) and achieving fairness of throughput among terminals.
Thus, in the conventional multicast service, the probability of losing a transmission frame due to interference, collision, time-varying channel characteristics, and the like, increases. In addition, the conventional multicast service does not use a data retransmission mechanism and does not support the exponential backoff and the physical layer rate adjustment. Thus, the reliability of the multicast traffic is inferior to the reliability of unicast traffic, and fairness of throughput among terminals is not readily achieved. In addition, in the conventional multicast transmission, generally, the physical layer rate is set to be low. Accordingly, transmission throughput of the multicast service is low.